1. Field of the Invention
The present invention relates to blocking oscillators adapted for switching inductive loads and, more particularly, to a blocking oscillator that is resistant to transient voltage fluctuations.
2. Description of the Prior Art
Electromagnetic fluid pumps are used for a wide variety of applications including supplying fuel to internal combustion engines. As a result, they are required to operate over a wide range of ambient temperatures and endure relatively large voltage fluctuations. They are normally powered by the same electrical source providing electrical power to the engine and its accessories such as the ignition system, starter, and various other inductive devices. Operation of the starter, for instance, requires a large current which results in a substantial decrease in the output voltage of the electrical power source. Conversely, when the power to the starter is terminated, the energy stored in the starter windings is released (inductive kickback) which will cause a negative voltage transient in the electrical system beyond the nominal voltage level of the electrical power source. Another factor to be considered is load dumping in which a low impedance load is turned off or disconnected. This results in a temporary overvoltage condition which could also cause a failure of the circuit. Likewise, radio, ignition system and other devices connected to the electrical system of an internal combustion engine are sensitive to Radio Frequency Interference (RFI). Normally, filter devices such as diodes and R.C. networks are provided to surpress RF interference and prevent them from being adversely effected by the RFI interference generated by the other devices connected to the electrical system. However, these filter devices are not always totally effective and are also subject to failure, therefore, any electrical device connected to the electrical system should not be a source of RF interference. The primary factors to be considered are operation under low voltage input conditions, component failure under overvoltage conditions, and the generation of RFI. While most solid state devices can survive relative high overvoltage conditions in their nonconducting state, they are highly susceptible to damage if the overvoltage occurs while they are in the conductive state. These are commonly referred to as "current mode failures". Disclosed herein is an improved blocking oscillator circuit of the type generally disclosed in my prior U.S. Pat. No. 3,629,674 (December, 1971) and the H. P. Wertheimer, et al, U.S. Pat. No. 3,381,616 (May, 1968), which is an improvement thereon, and has a much higher tolerance to the voltage fluctuations than ever realized heretofore.